This invention relates to a method for automatically determining the location of a "hot carrier" in a multiple access data communication system. A hot carrier condition exists when a station's transmitter is stuck in a transmitting condition which can not be isolated from the common communication medium or channel without human intervention. The ability to have the stations automatically identify the location of the stuck transmitter is useful in at least two respects. It allows operating personnel to quickly locate and manually disconnect the faulty transmitter, so that the other stations may continue to use the medium, and it enables field repair personnel to quickly locate and repair the fault.
The invention can be implemented at very little cost in local area data communication networks which use carrier sense multiple access protocols with collision detection. Such systems, commonly termed CSMA/CD systems, are described in U.S. Pat. Nos. 4,063,220, granted to Metcalfe et al on Dec. 13, 1977, and 4,210,780 granted to Hopkins et al on July 1, 1980. In these systems the stations condition their access to a common medium or channel on sensing an idle signal condition on the medium. While transmitting, the stations keep their receiving circuits active and listen to the signals returning from the medium for detecting collisions (interfering transmissions by other stations). The facilities enabling the stations to listen while transmitting can be advantageously shared for performing the present "hot carrier detection" operation.
U.S. Pat. No. 3,825,897 describes a scheme for enabling stations in a system adapted particularly for the communication of alarm events (e.g. tripped burglar alarms), from outlying stations to a central monitoring facility, to continue to communicate such events when one of the stations has a "runaway transmitter". The stations do not assist in locating the fault. If the line is continuously busy, a station having an event to transmit operates after a timeout to override its line lockout circuitry and transmit its signal (in potential interference with the signal being emitted from the stuck transmitter). In this system, alarm event signals are represented by discrete pulses with long time separations. Thus, it is possible for the signal sent by an overriding station to be recognized at the central station, particularly if it is sent repeatedly. This would not have application to contemporary systems in which the data pulses are very closely spaced in time.